The following discussion of the prior art is intended to present the invention in an appropriate technical context and allow its significance to be properly appreciated. Unless clearly indicated to the contrary, however, reference to any prior art in this specification should be construed as an admission that such art is widely known or forms part of common general knowledge in the field.
U.S. Pat. No. 6,875,867 B2 (“the '867 patent”) discloses a process for the preparation of chiral diol sulfones of general Formula (1-A),
wherein R is as shown below:
wherein R4a is preferably aryl such as phenyl.
U.S. Pat. No. '867 is directed to the synthesis of a chiral sulfone intermediate (prepared Kaneka alcohol preferably via triflate and sulfide intermediates) which is used in preparing a dihydroxy acid HMG CoA reductase inhibitor or lactone thereof. The invention also encompasses the process involving Juila-Kocienski olefination reaction wherein the chiral sulfone intermediate is reacted with a carboxylaldehyde to form the desired trans intermediate which may be converted to the final HMG CoA reductase inhibitor.
U.S. Pat. Nos. 6,344,569 and 5,278,313 disclose a process for the preparation 1,1-dimethylethyl(3R,5S)-6-chloro-3,5-dihydroxyhexanoate from (S)-4-chloro-3-hydroxybutyric acid ester (e.g. the specification of U.S. Pat. No. 1,723,728) as shown in scheme-1, which is the key intermediate for the synthesis of chiral diol sulfone of general Formula (1-A). U.S. Pat. No. 6,344,569 further discloses the process for the conversion of 1,1-dimethylethyl(3R,5S)-6-chloro-3,5-dihydroxyhexanoate to 1,1-dimethylethyl-(4R,6S)-6-chloromethyl-2,2-dimethyl-1,3-dioxane-4-acetate in example-3. Both the patents are provided herein as reference in its entirety.

International (PCT) Publication WO 2008/644243 A2 provides a process for the preparation of statin derivatives or its pharmaceutically acceptable salts thereof of general Formula through novel intermediates,
wherein ‘═’ denotes single or double bond and Y is H, Na, K, Mg, Ca and R is as defined above from compounds of HMG-CoA reductase residues like a to i.
Particularly, for instance rosuvastatin calcium salt of Formula 9a is prepared by employing Julia-modified olefination,
which comprises of the following steps:reacting the sulfone compound of Formula-2a
with n-butyl amide compound of Formula-3b
in presence of a suitable alkali metal carbonate like potassium carbonate in a polar aprotic solvent like dimethyl sulfoxide, followed by isolation using cyclohexane to provide corresponding olefin compound of Formula-4a.

The olefin compound of Formula-4a can be converted to dihydroxy compound of Formula-4x by deprotecting the acetonide protection with acid hydrolysis followed by preparation of tertiary butylamine salt of rosuvastatin of Formula-5a and finally converting the salt to rosuvastatin calcium of Formula-9a as shown in Scheme-2.

The similar process is employed for the preparation of other statin derivatives by changing the starting material via Julia-modified olefination process.
The process for the preparation of chiral diol sulfones of general Formula 1 as reported in the prior art involves two many steps starting from (S)-4-chloro-3-hydroxybutyric acid ester via kaneka alcohol. The process involves formation of triflate compound followed by condensation with thiol derivative to give thioether derivative. The S-oxidation of thioether derivative provides Julia Kocienski type intermediate.
International (PCT) Publications WO 2006/126035 A2, WO 2007/125547 A1 and WO 2010/023678 A1 discloses an alternative process for the preparation of rosuvastatin calcium.
U.S. Pat. No. 6,844,437 B1 and U.S. Pat. No. 6,784,171 B2 provides a process for the preparation of tert-butyl (E)-(6-{2-[4-(4-fluorophenyl)-6-isopropyl-2-[methyl(methylsulfonyl)amino]pyrimidin-5-yl]vinyl}-(4R,6S)-2,2-dimethyl[1,3]dioxan-4-yl)acetate which comprises reaction of diphenyl[4-(4-fluorophenyl)-6-isopropyl-2-[methyl)methylsulfonyl)-amino[pyrimid-in-5-ylmethyl]phosphine oxide with tert-butyl 2-[(4R,6S)-6-formyl-2,2-dimethyl-1,3-dioxan-4-yl]acetate in the presence of a strong base.
The present inventors have found that use of chiral diol sulfone intermediate of general Formula (1) provides the better alternative to prepare rosuvastatin calcium in high yield and purity. The present process avoids long multistep synthesis processes as disclosed in U.S. Pat. Nos. 6,844,437 B1; 6,784,171 B2; and 6,875,867 B2 and WO 2008/644243 A2.
The disadvantages of the prior art include multiple steps resulting in difficult preparations, the use of expensive reagents and reagents that are difficult to use on a commercial scale. This makes the process more difficult to perform, and time consuming and expensive technique for purification.
Accordingly, there is a need for an improved process for the preparation of HMG-CoA reductase inhibitors like rosuvastatin and intermediates thereof that eliminates the problems of the prior art on a commercial scale in a convenient and cost efficient manner.